1. Field of the Invention
The present invention relates to a nickel alloy having an excellent corrosion resistance, which is used for pipes, structural materials and structural members, such as bolts or the like, in a nuclear power plant or in a chemical plant. The present invention also relates to a method for manufacturing such a nickel alloy.
2. Description of the Related Art
A nickel alloy having an excellent corrosion resistance, such as Alloy 690 (60Ni-30Cr) or the like, is traditionally used for pipes, structural materials and structural members, such as bolts or the like, in a nuclear power plant or in a chemical plant. A typical example of corrosion encountered in nickel alloys is the intergranular stress corrosion cracking (IGSCC). In order to ensure safety in nickel alloys, it is important to prevent the occurrence of the IGSCC.
As a measurement method for enhancing the corrosion resistance of such a nickel alloy or a steel including high Ni content, instead of the composition designing method in which one or more elements having a high corrosion resistance are added to the base metal, a heat treatment either for suppressing the occurrence of chromium depletion layers in grain boundaries to strengthen the grain boundaries or for precipitating Cr carbides in grain boundaries, is conventionally employed as a preventive measure in the manufacturing technology.
For instance, in Japanese Patent Publication No. 2983289 a thermomechanical process for enhancing the intergranular corrosion resistance is disclosed in order to improve the resistance against IGSCC for austenite stainless alloy, wherein the number of “specialized” grain boundary portions is increased by controlling the cold working process and annealing process. In the process, the corrosion resistance can be enhanced by increasing the coincidence boundary rate up to 60% or more.
The coincidence grain boundary used herein means a grain boundary in which several lattice points in one of two adjacent grains are coincident with lattice points in the other of the adjacent grains, when the former grain is rotated around a crystallographic axis relative to the latter grain. In such a coincidence grain boundary, the lattice arrangement is highly coherent and the grain boundary energy is smaller as compared with that in the normal grain boundaries. A typical example of such a coincidence grain boundary is the twin boundary.
A grain boundary having a small difference in the crystallographic orientation between the adjacent grains is called as a low angle boundary (in this case, the difference is normally 15 degrees or less). Moreover, a grain boundary other than the above-mentioned grain boundaries, i.e., the coincidence grain boundary and the low angle boundary, is called as a random orientation boundary.
In an austenite stainless alloy disclosed in Japanese Patent Publication No. 2983289, almost all of the coincidence grain boundaries are twin boundaries. In the normal alloy structure, the grains are rarely constituted by twin grain boundaries, and each of twin grain boundaries is usually surrounded by random orientation boundaries. Regarding the coincidence grain boundary, it is effective to suppress the corrosion for grain boundaries on the surface. However, in the case when the stress corrosion cracking develops preferably on the random orientation boundaries, the coincidence grain boundary is insufficient for suppressing the development of the cracking.
It can be stated, therefore, that the process method proposed in Japanese Patent Publication No. 2983289 ensures insufficient resistance against the IGSCC. Moreover, Japanese Patent Publication No. 2983289 does not explicitly refer to any effect of the low angle boundary on the corrosion resistance in the alloy.
On the other hand, focusing on the low angle boundary as an index representative of the feature of the grain boundary, Japanese Patent Application Laid-open Publication No. 5-59473 discloses an invention of a Ni base super alloy, which has a low angle boundary resistance property and is capable of being cast as a single crystal product which is practically useful in using as a high temperature structural material for a gas turbine engine of an air plane, in particular for a rotary blade.
However, in accordance with the knowledge on the low angle boundary in Japanese Patent Application Laid-open Publication No. 5-59473, it is noted that the low angle boundary has a coherent lattice arrangement and therefore a smaller surface energy than a high angle boundary, and it is further noted that the low angle boundary has a smaller magnitude in the effect on the mechanical and chemical properties as compared with the high angle boundary, so that it is more favorable for usage as compared with the high angle boundary. Nevertheless, the actual effect and advantage that the low angle boundaries among the grain boundaries influence the properties of the nickel alloy is obscure in the above publication.
Moreover, Japanese Patent Application Laid-open Publication No. 2002-1495 deals with a high angle boundary as an index representative of the feature of the grain boundary, and discusses the rate of the high angle boundaries. In the publication, it is described that the surface quality of an austenite stainless steel sheet can be enhanced by controlling the rate of the high angle boundaries among all of the grain boundaries in the crystal structure so as to become more than 85%.
The austenite stainless steel sheet disclosed in Japanese Patent Application Laid-open Publication No. 2002-1495 is used as a material for an interior in a building or a raw material for a home appliance. This type of the stainless steel causes problems to be provided from consumers regarding the surface smoothness and/or the surface glossiness, so that the surface quality is controlled so as to suppress the occurrence of surface defects, in particular so-called roping. In view of this fact, the material, with which Japanese Patent Application Laid-open Publication No. 2002-1495 deals, is not such an alloy having an excellent corrosion resistance, in particular such an alloy having an excellent resistance against the IGSCC, as used for pipes, structural materials and structural members in a nuclear power plant or in a chemical plant.
As described above, in the process proposed by Japanese Patent Publication No. 2983289, the corrosion resistance can be enhanced by increasing the relative number of coincidence boundaries, since the coincidence boundary is effective for suppressing the corrosion of the grain boundaries in the vicinity of the surface. However, in the case when the stress cracking develops preferentially in the random orientation boundaries, no sufficient resistance against IGSCC can be ensured. In addition, there is no description on the low angle boundaries regarding the corrosion resistance of the grain boundaries in the above publication.
Japanese Patent Application Laid-open Publication No. 5-59473 and Japanese Patent Application Laid-open Publication No. 2002-1495 disclose the knowledge respectively regarding the high angle boundary and the low angle boundary as an index representative of the feature of the grain boundaries. However, in Japanese Patent Application Laid-open Publication No. 5-59473, it is not disclosed what feature can be actually obtained therefrom. Furthermore, in Japanese Patent Application Laid-open Publication No. 2002-1495, pipes, structural materials and structural members, which have an excellent corrosion resistance, are not dealt with therein.